Umm, not as expensive as putting a roof over another room. Just about anytime you can go upwards, it's less expensive than going outwards.
The walls must be considered and upgraded, the floor system has to be considered and supported (in a garage, you're looking at span issues), then there is the diffuculty involved in working at the additional height.
May not be so cheap that you can't resist building it, but less expensive than building both a garage and an extra room.
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